Conventionally, blades of rotor of cooling fan are thermoplastic substances formed by injection molding. It is often found that not all blades mounted on fan have the same weight due to the uncontrollable amount of variation in the manufacturing process. As such, shaft of fan may deflect to one side in high speed rotation which in turn cause a rough rotation of shaft and a uneven contact between shaft and bearing. This situation and others such as noise and overworn are even worse in cooling fan with a single ball bearing provided. This significantly shortens the useful life of cooling fan.